Sill Slope and Pan Flashing Explained

By the time the old frame is out of the wall, the sill tells the whole story, and the intervals window installation austin tx love my cc uses for checking an opening are shorter than the ones a manual suggests, because the sill is where the water ends up. An installer in Austin who finds a flat or backward sill at this point has a choice to make: build it right, or hide the problem under a new window.

What the slope is for

The rough sill at the bottom of the opening should fall toward the outside, and a quarter inch over the depth of the wall is a common target. That small fall sends any water that gets past the frame out onto the siding instead of back into the wall. A level sill lets water sit against the wood, and a backward one drains it into the house.

The pan flashing under the frame

A pan flashing is a waterproof tray built into the bottom of the opening. It can be a formed metal piece, a flexible membrane or a taped sill pan. It runs under the frame and turns up at both ends, and the raised edges are called end dams. Without end dams the water runs sideways to the jamb and into the wall.

Why a vinyl window needs one as much as a wood window

A vinyl window does not rot, but the framing around it does. An aluminum window frame will not swell, yet the stud and sheathing behind it soak up whatever water slips past. A fiberglass window holds its shape well and still depends on the pan flashing to keep the wall dry.

Checking from the outside

Look at the bottom edge of the frame. Water should drip off cleanly, and the siding below should be free of dark streaks. A line of caulk along the bottom of a sill with no gap means the installer closed the drainage path. In Mueller and Travis Heights, where newer builds use stucco and fiber cement, that gap is often the first thing a painter fills in.

Checking from the inside

Run a level along the interior stool. A bubble that drifts toward the room means the rough opening was never sloped, or the house has settled since. Austin has expansive clay soil, and foundations move with wet and dry cycles, which tips a sill that once sat true in a lot of Austin, TX houses. A double pane unit set in a tipped sill also sees uneven load on its edge seal.

Fixing it means opening the wall around the window, shaping the sill with a tapered piece of wood or a pre-sloped sill dam, and lapping the membrane up the sides before the new frame goes in.